Proceedings
of The World Avocado Congress III, 1995 307 - 310
FRUIT
MINERAL CONCENTRATIONS AND POSTHARVEST QUALITY OF 'HASS' AVOCADOS.
T. G. Thorp, D. Hutching, T. Lowe, and K.B. Marsh
The
Horticulture and Food Research Institute of New Zealand Ltd.
Private
Bag 92 169
Auckland
New Zealand.
Avocado,
Persea americana, fruit minerals, fruit maturity, calcium
Abstract
New Zealand
avocado orchards (cv Hass) were surveyed to determine if an imbalance in fruit
mineral concentrations (Ca, Mg, and K) was affecting fruit quality. Only one
orchard had fruit with low Ca concentrations compared with overseas standards.
These fruit had more vascular browning and flesh browning than fruit with high
Ca concentrations. Also, fruit Ca concentrations were lower and the incidence
of vascular browning higher in fruit that were more mature at harvest.
Introduction
Several
preharvest factors will affect the postharvest quality of avocado fruit (Hofman
and Smith, 1994). Important among these are fruit mineral concentrations,
especially the levels of Ca, Mg and K. 'Fuerte' fruit with low Ca
concentrations are more susceptible to the physiological disorders vascular
browning and flesh browning (Chaplin and Scott, 1980; Bower and Cutting, 1988).
Both of these disorders can be accentuated by cold storage. A similar
relationship has been suggested for 'Hass' fruit, but has yet to be
demonstrated (Cutting and Bower,
1992).
Our objective was to
establish benchmark levels for Ca, Mg and K in New Zealand- grown 'Hass' avocados,
and to examine the relationship between these fruit minerals and postharvest
fruit quality.
Materials and Methods
Fruit at similar
maturities were harvested from 'Hass' trees on 11 orchards distributed over the
northern half of New Zealand. On each orchard, two fruit were selected from
each of 30 trees. Harvested fruit were weighed, packed at random into single
layer trays, stored for 3 weeks at 6 ēC, then transferred to 20 ēC until ripe.
When ripe, fruit were weighed, assessed for external rots, cut into quarters
longitudinally, the skin peeled away and the flesh assessed for stem end and
body rots, vascular browning and flesh browning using a severity rating scale
from 0 (absent) to 2 (severe).
Fruit maturity and mineral
concentrations were determined from flesh samples (approximately 5g per fruit).
Each sample was weighed, freeze-dried and re-weighed to give % dry weight
(%dw), and converted to %dw at harvest by correcting for weight loss during
storage and ripening. The dried tissue was then ground, and a 0.2 g subsample
digested at 120 ēC for 120 mins in 2 mls concentrated HNO3 After
cooling to 60 ēC, a total of 2 mls 30% H2O2 was added in
0.5 ml aliquots, and the digestion continued for 60 mins at 60 ēC, followed by
90 mins at 120 ēC. Digested samples were made up to 25 ml with distilled water
(0.2% La), and analyzed for Ca, Mg and K by atomic absorption (AA). Soil and
leaf samples were also collected from each orchard in March (late-summer) and
sent to a commercial laboratory for mineral analyses.
Results
At harvest, average fruit
weights were between 199 and 254g, and fruit maturities were between 24 and
37%dw. Most fruit ripened within 7 days of being removed from cool storage.
Fruit from Orchard 3 took the longest (P<0.001) to ripen and they had the
lowest (P<0.001) dry weights at harvest (10.2 days and 24%dw, respectively).
Fruit from Orchards 1 and 7
had the lowest (P<0.05) Ca concentrations with 28mg/100g dw and 25mg/100g
dw, respectively (table 1). Mean fruit Mg concentrations were between 91 and
113 mg/100g dw, and K concentrations were between 1126 and 1608 mg/100g dw.
There was no correlation between fruit mineral concentrations and soil and leaf
concentrations (unpublished data).
Only fruit from Orchard 7 had
significant (P<0.001) levels of vascular browning and flesh browning (table
1). On this orchard, fruit with low Ca concentrations had more
vascular browning than fruit with high Ca concentrations (figure 1). Also,
fruit Ca concentrations were lower in fruit which were more mature (higher %dw)
at harvest (r = -0.69; P<0.05). Fruit Mg and K concentrations were not
related to the incidence of physiological damage. The most common fruit rots
observed were the anthracnose fungi Colleototrichum gloeosporioides and C.
acutatum, and the soft rot fungus Botryosphaeria parva (Shelly
Forbes, HortResearch). The percentage of fruit with severe rots ranged from 2%
to 38% at Orchards 4 and 1, respectively (table 1). There was no correlation
between fruit mineral concentrations and the incidence of fruit rots, but the
percentage of fruit with severe rots increased with increasing fruit maturity
at harvest (r=0.72, P<0.001).
Discussion
Only one New Zealand orchard
had average fruit Ca concentrations less than those reported for South African
fruit; 25 and 28 mg/100g dw, respectively (Cutting and Bower, 1992). On this
orchard (Orchard 7), fruit with low Ca concentrations had more vascular
browning and flesh browning than fruit with high Ca concentrations. This suggests
that there is a threshold between 25 and 28 mg Ca /100g dw for, symptoms of Ca
deficiency to appear, as fruit in New Zealand and South Africa with 28 mg
Ca/100g dw had practically no vascular or flesh browning. Fruit Ca
concentrations at Orchard 7 were lower and the incidence of vascular browning
higher in fruit that were more mature at harvest. Cutting and Bower (1992) also
reported decreasing Ca concentrations with increasing fruit maturity in
'Fuerte'. If fruit Ca concentrations are known to be low, then it would be
important to harvest these fruit earlier than fruit from orchards with
sufficient Ca.
It is not clear
what caused the low levels of Ca in fruit from Orchard 7, as low fruit Ca did
not correlate with low Ca concentrations in the leaves or soil. Avocado flowers
have high transpiration rates and trees can suffer from excessive water loss
during heavy flowering (Whiley et al., 1988). This may have occurred at
Orchard 7, and resulted in low Ca concentrations in fruit from this orchard
(Bower and Cutting, 1988). Alternatively, water stress may have made these
fruit more susceptible to physiological damage, independent of Ca
concentration.
We did not find a direct link
between fruit mineral concentrations and postharvest fruit rots. However, fruit
that were more mature at harvest ripened more quickly than fruit that were less
mature, and they were more susceptible to rot development, irrespective of
fruit mineral concentration. We found no evidence that low Ca concentration
caused fruit to ripen more quickly (Cutting and Bower, 1992).
In conclusion, Ca nutrition
and harvest maturity are important determinates; of postharvest fruit quality
in New Zealand-grown 'Hass' avocados. Harvesting fruit too early increases the risk
of uneven ripening although there could be benefits from an extended ripening
period. Harvesting fruit too late means that fruit are quick to ripen, but they
are more susceptible to rots and physiological damage, especially if they have
low Ca concentrations.
References
Bower, J.P., and Cutting,
J.G.M., 1988. Avocado fruit development and ripening physiology. Hort. Reviews.
10:229-271.
Chaplin, G.R., and Scott,
K.J., 1980. Association of calcium in chilling injury susceptibility of stored
avocados. HortSci. 15(4) :514-515.
Hofman, P.J., and Smith,
L.G., 1994. Preharvest effects on postharvest quality of subtropical and
tropical fruit. In Postharvest handling of tropical fruits: ACLAR Proc.
50:261-268.
Whiley, A.W., Chapman, K.R.
and Saranah, J.B. 1988: Water loss by floral structures of avocado (Persea
americana cv. Fuerte) during flowering. Aust.J.Ag.Res.39 :457-467.
Acknowledgements
We acknowledge the financial
assistance of the NZ Avocado Growers' Assn. and the NZ Foundation for Research,
Science and Technology.